Semiconductor devices are formed on semiconductor substrates using a manufacturing process that typically includes several wet chemical processing operations. The wet processing operations include cleaning operations, stripping operations and etching operations in which the chemicals of the chemical bath react with a material such as a film or other material being etched or removed. The use of wet chemical benches to perform these operations has been and continues to be a standard in the semiconductor manufacturing industry.
As devices become more complex, feature sizes become more miniaturized and film thicknesses become reduced, it becomes increasingly important to eliminate contamination and to process the substrates in a uniform and repeatable manner. For example, as film thicknesses decrease and substrate sizes increase, it becomes increasingly challenging to completely and uniformly remove the thin film in a short time without attacking any underlying structures, i.e. so that the entire film clears at the same time. In order to effectuate a clean, uniform and repeatable wet chemical processing operation, the wet chemical processing bath should desirably be maintained in the same condition throughout the lifetime of the bath. For example, chemical concentrations and bath temperatures must be maintained uniformly throughout the bath and constant in time. Contaminants must be minimized. The etch rate or stripping rate of a bath is dependent upon the aforementioned conditions of the bath and it is very critical to maintain an etch or stripping rate that is constant in time as well as uniform throughout the bath.
One challenge in maintaining a constant etch or stripping rate, is that the etch or stripping rate is critically dependent on the concentration of the reactive chemical species in the bath, and this reactive chemical species is consumed during the etching/stripping operation. As an example that will be discussed below, the concentration of phosphoric acid determines the etching/stripping rate of silicon nitride. As such, it is of critical importance to maintain a constant concentration of the critical reactive chemical species during the period in which that chemical is being consumed by reacting with the semiconductor substrate.
In conventional processing, chemical concentrations, chemical bath temperatures and other characteristics are monitored after a number of semiconductor substrates have been processed, i.e., after several “runs”. Naturally, the concentration of critical reactive chemicals diminish as they are consumed in time, and must be replenished. In some examples, a spiking technique is used to spike the chemical bath by adding a high concentration of the species that has become depleted in time. This spiking procedure, however, is done infrequently and allows the chemical bath concentration to undesirably change run-to-run as it becomes depleted and also causes the chemical bath concentration to undesirably spike above desired concentration levels when the replenishing occurs.
It would be therefore desirable to maintain the wet chemical bath to have constant chemical concentration levels, and therefore constant etch/strip rates, in time. This is especially true for the reactive chemical species.
Additionally, the reaction product between the reactive chemicals in the wet chemical bath and the material being removed, often produces precipitates in the wet chemical bath. It is critically important to prevent these precipitates from settling on the surface of the substrate being processed as they can block the complete removal of the film being etched or removed and they can present other contamination problems if they remain on the surface. It would therefore be desirable to monitor the concentration of these solid precipitates and efficiently remove them to maintain sufficiently low contamination levels in the bath.
Silicon nitride is a dielectric material that is very frequently used in many applications in the manufacture of semiconductor devices. A film of silicon nitride is typically formed over a semiconductor substrate upon which semiconductor devices are being fabricated. The silicon nitride film may be patterned using an etching process that includes phosphoric acid. In other embodiments, the remaining patterned portions of the silicon nitride film are stripped. In still other exemplary embodiments, a complete Si3N4 film may be removed by stripping. The wet chemical etching/stripping of silicon nitride has traditionally been done using hot phosphoric acid (H3PO4) at elevated temperatures. The basic chemical reactions that model the removal of silicon nitride by phosphoric acid are:

The silicon nitride etch/strip process is heavily influenced by process parameters including H3PO4 and silicon concentration, as evidenced by silica, SiO2 precipitation, temperature of the etch bath and the bath life of the hot phosphoric bath. The oxide (silicon dioxide, SiO2) etch rate is also affected by process conditions, in particular the silicon concentration in the etching bath. The oxide etch rate becomes dramatically lower as the Si concentration in the bath increases. The Si concentration therefore directly affects the silicon nitride:silicon oxide etch selectivity. If the silicon oxide etch rate increases with respect to the silicon nitride etch rate, underlying silicon oxide structures may be destroyed during the silicon nitride etch or stripping process.
It is therefore desirable to maintain the bath conditions such as the H3PO4 and silicon concentration and the bath temperature, at constant levels so as to produce constant etch rates and etch selectivities, and process repeatability. As bath life increases and the above reactions occur, however, these parameters may undesirably vary. It can be seen that it is critical to maintain and control the silicon, Si, concentration in order to maintain constant silicon nitride and silicon oxide etch rates and silicon nitride/oxide etch selectivity. Still referring to the chemical reaction equation, literature indicates that the oxide precipitate, i.e., the dehydration of Si3O2 (OH)8 to form SiO2 and water, occurs after reaching saturation solubility at about 120 ppm at 165° C. Different temperatures have other saturation solubility levels but, at any rate, it is desirable to maintain a SiO2 concentration that is low enough to avoid precipitation. The generation of oxide precipitates results in an undesirable particle source.
One known procedure for maintaining a wet silicon nitride etch process is to allow the silica to precipitate and to remove the precipitate in a process that involves decreasing the temperature. One silica extraction system is designed to remove the generated silica using lower temperature H3PO4 with a high Si concentration. Shortcomings of this procedure include the difficulty in maintaining a high extraction efficiency for precipitated silica, which may melt into the H3PO4 solution again if not removed quickly and if allowed a long reaction time.
Another procedure as provided in commonly-owned U.S. patent application Ser. No. 11/627,030 filed Jan. 25, 2007, the contents of which are hereby incorporated by reference as if set forth in their entirety, is to monitor the silicon in solution by diluting and cooling a phosphoric acid sample with deionized water. This procedure requires extra monitor systems as well as dilution and cooling capabilities.
Improved methods and systems for maintaining a wet chemical bath to have constant chemical concentration levels, and therefore constant etch/strip rates in time are therefore desirable.